1. Field of the Invention
The present invention relates to an amplification-type solid state imaging device.
2. Related Background Art
Conventional solid state imaging devices are roughly clasified into the CCD-type solid state imaging devices end the amplification-type imaging devices. In the CCD-type imaging device each unit picture element is comprised of a buried photodiode (hereinafter simply referred to as a BPD) serving as a photoelectric conversion device for converting the incident light to a signal charge and a transfer device for transferring the signal charge for reading. The BPD is in the form of a structure in which a shallow diffused region of the opposite conductivity type (i.e., the N-type in the case of the P-type photodiode and the P-type in the case of the N-type photodiode) is formed on a P-N junction region.
On the other hand, in the amplification-type solid state imaging device each picture element is comprised of a transistor such as a MOS-type static induction transistor (hereinafter simply referred to as a MOS-SIT), a junction-type field effect transistor (hereinafter simply referred to as a JFET) or a bipolar transistor and a photoelectric conversion device is composed of a MOS diode or a P-N junction diode forming a part of the component elements of the corresponding transistor. However, the above-mentioned conventional solid state imaging devices have the following disadvantages:
Firstly, the CCD-type imaging device has the disadvantage of being low in sensitivity as compared with the amplification-type imaging device due to the fact that the signal charges are transferred without amplification separately with respect to the individual picture elements, although the device involves the following merits owing to the use of the buried photodiodes for the photoelectric conversion devices:
(a) the quantum efficiency is high; PA1 (b) there is less occurrence of persistence; and PA1 (c) there is the less dark current and hence there is the less fixed pattern noise (hereinafter referred to as an FPN) caused by variations of the dark current.
On the other hand, in the amplification-type imaging device the photoelectric conversion is effected by the diode forming a part of the components of each amplification-type transistor and thus it is impossible to obtain such excellent photoelectric conversion characteristic as that of the CCD-type imaging device employing the buried photodiodes for photoelectric conversion. For instance, the M0S diode has the disadvantage of not only deteriorating the quantum efficiency but also increasing the surface leak current due to the low light transmittance of the polysilicon layer forming the gate electrode and there is the problem of increasing the FPN. Also, the P-N junction diode is subject to the occurrence of persistence dye to the incompleteness of its resetting operation. To decrease the density of the photodiode region in an attempt to realize a complete resetting operation rather gives rise to an inconvenience of increasing the surface leak current.